$addFields (aggregation)

The $addFields stage in the aggregation pipeline is used to add new fields to documents. It can also be used to reset the values of existing fields. This stage is particularly useful when you need to create new fields based on existing data or modify existing fields within your documents.

Syntax

The basic syntax for the $addFields stage is as follows:

{
  $addFields: {
    <newField1>: <expression1>,
    <newField2>: <expression2>,
    ...
  }
}

Parameters

Description
newField1 The name of the new field to add or the existing field to modify.
expression1 The expression to compute the value of newField1.

Example(s)

Example 1: Adding a New Field

Suppose we have a collection named stores with documents similar to the provided JSON structure. To add a new field totalDiscountEvents that counts the number of promotion events, we can use the following aggregation pipeline:

db.stores.aggregate([
  {
    $addFields: {
      totalDiscountEvents: { $size: "$store.promotionEvents" }
    }
  }
])

Sample output

[
  {
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "store": {
      "name": "Downtown Store",
      "promotionEvents": ["Summer Sale", "Black Friday", "Holiday Deals"]
    },
    "totalDiscountEvents": 3
  }
]

Example 2: Modifying an Existing Field

If we want to add a field totalStaffCount that sums up the full-time and part-time staff, we can use:

db.stores.aggregate([
  {
    $addFields: {
      totalStaffCount: {
        $add: ["$store.staff.totalStaff.fullTime", "$store.staff.totalStaff.partTime"]
      }
    }
  }
])

Sample output

[
  {
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "store": {
      "name": "Downtown Store",
      "staff": {
        "totalStaff": {
          "fullTime": 12,
          "partTime": 8
        }
      }
    },
    "totalStaffCount": 20
  }
]

Example 3: Adding Nested Fields

To add a nested field location.coordinates that combines latitude and longitude into an array, use:

db.stores.aggregate([
  {
    $addFields: {
      "store.location.coordinates": ["$store.location.lat", "$store.location.lon"]
    }
  }
])

Sample output

[
  {
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "store": {
      "name": "Downtown Store",
      "location": {
        "lat": 47.6097,
        "lon": -122.3331,
        "coordinates": [47.6097, -122.3331]
      }
    }
  }
]

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